Tanks that store liquids, especially crude oil, have to be periodically cleaned. While the discussion of the prior art that follows is primarily directed to the cleaning of shore side tanks for the storage of all liquids, the reader will understand that the following invention relates to flat bottom liquid storage tanks.
For example, oil storage tanks, especially those tanks that typically collect and store crude oil, accumulate sludge. This sludge constitutes both the gravitationally classified heavier petroleum products as well as heavy impurities that are entrained in the crude oil as it is delivered to the storage tank. Such heavy impurities can include solids entrained in the crude oil as it is extracted from the production site, rust and waste materials accumulated during transport and handling of the crude, and even debris thrown or inadvertently dropped into either the crude or the myriad of containers receiving and handling the crude before its arrival in the storage facility.
An example of how sludge preferentially occurs in shore side crude oil storage tanks can be understood from the unloading of transporting oil tankers. Oil tankers usually have systems for cleaning their oil transporting bottoms. These systems are typically activated during off loading. Naturally, the sludge and debris accumulated by such systems ends up within the shore side crude oil storage tank.
The size of such oil storage tanks constitutes another problem. Typically, such tanks are large--tanks are known that are in the order of 300 feet in diameter. Dependent upon the interval between cleaning and the type of crude handled, sludge can accumulate in great volume. It is known for sludge to be in the order of 20 feet thick, although a more reasonable and normal accumulation of such sludge is in the order of 2 feet of depth.
The accumulation of such sludge requires that eventually the sludge must be removed. Sludge can accumulate to the point where it interferes with the designed capacity of the tank. Additionally, the sludge can cause problems; an example of the problems caused by the accumulation of such sludge is typified by the interaction of an accumulation of sludge with the top of the tank.
Crude oil storage tanks are typically equipped with floating tops to minimize the gas-air space above the stored material in the tank. When the tanks are pumped dry, the tops descend and are designed to rest on legs that protrude downward into contact with the bottom of the tank. Unfortunately, when the tanks become full of sludge at their bottoms, the function of the tops in settling on their legs is interfered with. Specifically, the tops settle onto the sludge and become cocked with respect to the tank. When this occurs, serious damage to the floating top of the tank, as by bending at the floating top of the tank can occur.
The bottom of an oil tank with accumulated sludge is a hostile environment containing many conditions that render the removal of the sludge and the cleaning of the tank bottom difficult. First, the tank contains a potentially explosive atmosphere. Care must be taken not to initiate an explosion precipitating spark. Further, safety requires that the atmosphere interior of the tank be rendered non explosive. This is presently accomplished by venting.
Even venting is not without its problems. Air pollution regulations generally prohibit venting of such crude oil storage tanks to the atmosphere, except where no other practical alternative can be found. At present, venting of such tanks during cleaning is permitted as a so-called exception to laws relating to air pollution. If a process or apparatus for the cleaning of such tanks could be found that did not require such venting, pollution could be reduced.
Further hazards present in the tank cleaning process are present. Tank bottoms can be and usually are fragile. While such bottoms are designed to handle the static fluid pressures of liquids, they are not designed to receive the loading cause by vehicles passing over their bottom surfaces. Heavy vehicles cannot be used for such tank cleaning. Such heavy vehicles traversing such tank bottoms can cause damage to the tank bottoms and even break through the bottoms themselves. Further, a main reason for the requirement of removal of the sludge at the bottom of the tank can be the necessity for replacement of the tank bottom. Such tank bottoms often have to be replaced due to corrosion--either cause by electrolysis, moisture, or even acidic components within the stored liquids. Clearly, where the bottom of the tank is so fragile that it must be replaced, breakage of the tank bottom with the accompanying leakage and loss of contained materials is to be anticipated.
Lastly and most importantly such tanks bottoms are slippery. What is present on the bottom of the tank is a smooth metallic surface lubricated by the wet sludge. This bottom surface constitutes a low traction environment where any vehicle can slip. Since only relatively light vehicles can safely be used over the tank bottoms, the problem of obtaining sufficient traction is especially aggravated.
It has been known in the past to clean such tanks by cutting a large access aperture in the tanks side. Such cutting is usually accomplished by utilizing a gas torch, mechanical saw or a high water pressure jet (in the order of 60,000 psi) to assure the absence of an explosion exciting flame or spark. Thereafter, once an access hole has been cut, the tank is vented and entry is made for cleaning. It is to be noted that in such cleaning, conventional manholes on the tank sides are usually not used; typically the access required for such cleaning exceeds the dimension of such manholes.
Transport of the extracted sludge has constituted another problem. As the sludge is usually a paste like substance having a, consistency less than that of a solid but too viscous to form a liquid, removal is anything but easy. Usually the sludge must be accumulated in a positive displacement device such as another open tank, and transported for disposal by the segregation of the sludge components.